As readers we only know what Harry notices/pays attention to, and although we have no idea what color Cho’s eyes are or what Zacharias Smith’s voice sounds like, we know a lot about Draco Malfoy. Consider. We know:
His hair is blond. But not just blond. It’s a particular, white-blond color that gleams in the sunlight.
And very sleek, but loose enough that it falls around his face if he moves around vigorously enough.
He has grey eyes. Not just grey, but specifically light grey.
Harry also frequently describes them as glittering or sparkling.
He presumably has a sweet tooth as he gets regular sweets packages from home delivered by his eagle owl.
He has very pale skin which takes on a grayish tinge when he’s sickly.
He tends to narrow his eyes when he’s angry.
He doesn’t blush when he’s embarrassed but he does flush - just enough to give his skin a faint pink tinge.
He tends to move his hands a lot when talking animatedly.
His speech has a very specific drawling cadence and tone that makes his voice easily recognizable (to Harry at least).
This is all in canon.
Meanwhile, even though Harry and Ron are together practically 24/7 for seven years, we only find out that Ron’s eyes are blue halfway through Deathly Hallows.
